Powering up a Prop using an LDO with enable

DiverBob

I have a battery-powered Prop using 2 3.6 v lithium-ion cells. These feed a MicroChip 1802T 5V LDO with an enable input. What I want to do is use a Prop output to feed the LDO enable to keep the LDO on and therefore programatically shutdown the whole circuit by bringing the enable low.

The problem I have is that there are only 3 monentary SPST switches (piezo switches) available to supply the initial hi enable signal to start up the regulator. I have included a simplified dwg of the circuit as it stands now. I'm trying to figure out the best method of applying the +7.2V voltage from the battery to the regulator enable pin without burning out the Prop pins and using the existing switches. The enable voltage only has to be held long enough for the prop output to take over - its the first thing in the SPIN code.

Preferably I'd like 2 switches pressed at the same time to trigger the enable but at this point even a single switch press for enable would be OK.

I've been going slightly nuts trying to figure this out but hopefully someone has encountered something like this before. The reg is great for battery power, very little current draw when disabled.

Mike Green

You'd use 2 diodes with 10K series resistors. One would be connected with the anode to the enable pin and the cathode to the Prop I/O pin. The other would be connected with the anode to a switch and the cathode to the enable pin. The other end of the switch goes to +7.2V.

When the switch is closed, the +7.2V pulls up the enable pin voltage through the diode and resistor to turn on the regulator. The resistor limits the current into the enable pin.

The Propeller normally has its I/O pin set to high-Z which should not affect the enable pin. When you set the Propeller I/O pin to output a low, the cathode of the diode is grounded and pulls down the enable pin turning off the regulator. When the regulator output drops below 2.7V, the Propeller will be forced into reset which will cause the I/O pin to become high-Z again. You probably will want a small capacitor (like 0.1uF ceramic) from the enable pin to ground to keep the enable pin high or low for a little while as everything shuts down or starts up.

Toby Seckshund

Is this the "chicken and egg" thing again?

The prop will have to be powered to power itself on. As there has been pointed out before there has to be a starting handle, this would only be able to power itself off. I am sure that a few months ago there were ccts to allow both ways, one was from parallax, if my feeble memory hasn't failed again.

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Sapieha

Hi DiverBob.

On Yours SCH You have 5V to Prop ... should it not be 3.3V

Regards
ChJ

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Nothing is impossible, there are only different degrees of difficulty.
For every stupid question there is at least one intelligent answer.
Don't guess - ask instead.
If you don't ask you won't know.
If your gonna construct something, make it·as simple as·possible yet as versatile as posible.


Sapieha

DiverBob

Thanks Mike for the idea, I'll try that out later. I've been experimenting with the regulator a bit, it seems pretty stable and with the enable pin it allows turning off the entire circuit. I had only gotten the current draw down to 14.3ma which drained the batteries in just over 5 weeks; this should allow the batteries to last a lot longer!

I like the Chicken and Egg analogy - I hadn't thought of that one.....

I showed only a partial circuit diagram, the Prop is powered by 3.3v but the 5v reg supplies the 3.3 reg. I just decided to simplify the circuit to show the essentials.